1. Field of the Invention
The present invention relates to a variable frequency oscillator, and more particularly to a variable frequency oscillator with a solid resonator device such as a ceramic resonator.
2. Description of the Prior Art
In the prior art, there has been proposed a variable frequency oscillator employing a solid resonator device, such as a ceramic resonator device. For example, Japanese electronics magazine "Nikkei Electronics" issued Jan. 30, 1984 and published by Nikkei-McGraw-Hill, Inc., discloses such a variable frequency oscillator on pages 151 to 157. See, in particular, FIG. 9 of that magazine. An equivalent variable frequency oscillator is also disclosed in the U.S. Pat. No. 4,587,500.
These conventional variable frequency oscillators are comprised of a resonant circuit, an amplifier connected to the resonant circuit for exciting an oscillation of the resonant circuit, and a variable reactance circuit connected in parallel to the resonant circuit. The variable reactance circuit has two amplification circuits operating respectively as positive and negative equivalent reactances. The positive equivalent reactance is regarded as an inductance (referred as positive inductance hereafter) or a negative capacitance. While the negative equivalent reactance is regarded as a capacitance (referred as positive capacitance hereafter) or a negative inductance.
The variable reactance circuit operates as one element for defining a resonant frequency together with the resonant circuit. Control signals are applied to the amplification circuits so that the equivalent reactance seen from the resonant circuit is adapted to be varied from a negative given value to a positive given value. That is, the variable reactance circuit may vary between an inductive state and a capacitive state. Accordingly, the oscillation frequency of the variable frequency oscillator may vary in a relatively wide range in accordance with the control signals through the variation of the reactance of the resonant circuit.
A resonant circuit current, i.e., a current flowing through a resonant circuit has an imaginary component due to the reactance of the resonant circuit, as well as a real component. As is well known, the resonant circuit current must have a reasonable amount of real component for maintaining a stable oscillation. If the real component of the resonant circuit current becomes insufficient, an amplitude of the resonant circuit current decreases or the oscillation is interruppted. On the other hand, if the real component of the resonant circuit current becomes too large, the resonant circuit current is distorted in its waveform or a parastic oscillation occurs. Therefore, it is desired that the real component of the resonant circuit current be maintained constant within the variable range of the oscillation frequency.
In the conventional variable frequency oscillator, however, a real component of a resonant circuit current flowing through the resonant circuit varies with the variation of the oscillation frequency. This is so since the resonant circuit current is a resultant vector of both an exciting amplifier current and a variable reactance circuit current, i.e., currents flowing through the exciting amplifier and the variable reactance circuit and applied to the resonant circuit therefrom. The variable reactance circuit current varies in accordance with the control signals. The real component of the resonant circuit current becomes too small when the reactance of the variable reactance circuit reaches a large positive capacitance or achieves a highly capacitive state. On the other hand, the real component of the resonant circuit current becomes too large when the reactance of the variable reactance circuit reaches a large negative capacitance or achieves a highly inductive state.
As a result, the conventional variable frequency circuit readily fails to maintain the oscillation in the former case. Or the conventional variable frequency circuit causes the undesired parasitic oscillation in the latter case.
The problems of the oscillation stoppage or the parasitic oscillation tend to occur because the variable frequency circuit is operated under a relatively low power supply voltage.